# Kinematics elevator problem

1. Mar 31, 2015

### erisedk

1. The problem statement, all variables and given/known data
A boy standing in an elevator accelerating upward throws a ball upward with a vo.
The ball returns to his hands after time t. The acceleration of the lift is

2. Relevant equations

3. The attempt at a solution
After time t, displacement of the ball = displacement of the lift
vo t - 1/2 gt2 = ?
What should the displacement of the lift be?
According to me, it should be ut + 1/2at2.
Problem is, I don't know what the initial velocity of the lift is, i.e. what u is.
What do I do?

2. Mar 31, 2015

### TSny

The quantity v0 is probably to be interpreted as the initial velocity of the ball relative to the elevator. You are writing equations relative to the ground. So, what is the initial velocity of the ball relative to the ground?

3. Mar 31, 2015

### Orodruin

Staff Emeritus
You are likely expected to understand that the velocity v0 is the velocity of the ball relative to the elevator. You can make any assumption on the velocity of the elevator, it should not affect the result - simply assume it is an undetermined velocity u.

4. Mar 31, 2015

### erisedk

Ok thank you!
So, displacement of the body in the frame of the elevator is 0 in time t, and so I can consider a pseudo force a on the elevator.
v0t - 1/2 (g+a) t^2 = 0
a= (2v0 - gt)/t

5. Mar 31, 2015

### Orodruin

Staff Emeritus
Correct.

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